This application is based on application No. H9-101704 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a mold, and more particularly, to a diffractive optical element such as a diffraction lens having on its surface a relief pattern like that of a blazed diffraction grating and a mold manufacturing method suitable for molding the same.
2. Description of the Prior Art
For example, as the taking lens for the single lens camera, a lens is known which uses a diffractive structure for correcting chromatic aberration. In the diffraction lens, as well known, at least one of the opposed surfaces of the lens body is curved in order to form a refractive area having a power, and a diffractive area is provided which has a power to reduce chromatic aberration led by the refractive area.
Normally, an optical element having a diffractive surface configuration typified by the above-described diffraction lens is manufactured by use of a mold for resin. As a method of manufacturing a mold for molding a diffractive optical element, various methods have recently been proposed which use cutting by use of a diamond tipped turning tool, so-called diamond turning. These conventional technologies all use diamond turning operations by linear interpolation in the directions of the Z- and X-axes. As an example thereof, U.S. Pat. No. 5,589,983 discloses a diffractive optical surface configuration manufacturing method where the cutting speed and the number of cutting rotations are clearly taught. This is not a disclosure as to processing conditions under which transmissive surfaces that effect diffraction can be so processed as to be substantially as smooth as a mirror surface depending on cutting conditions.
In processing a mold for molding an optical element having a normal spherical or aspherical curved surface relief configuration, a high-precision diamond tipped turning tool is used in which a point R surface called a corner R is approximately 1 mm in radius. By bringing the point R surface of the diamond tipped turning tool into point contact with work, removal processing of the contact points is performed.
In this case, a necessary curved surface configuration is typically formed by linear interpolation in the Z- and X-axes of the processor, and a desired processing configuration is obtained by controlling the X- and Z-axes of the processor so that the curved surface configuration such as an aspherical surface configuration is the locus of the contact points. However, on the processed surface formed by bringing such a diamond tipped turning tool into point contact with a mold material, a fine pattern is formed which comprises a geometric configuration with a level difference called a sculpture height.
The sculpture height depends on the configuration of the point R surface of the turning tool and the feed amount of the work per operation. That is, the greater the radius of the point R surface of the turning tool is and the smaller the feed amount of the work per operation is, the smaller the sculpture height is. For example, when processing is performed under normal processing conditions by use of a diamond tipped turning tool with a point R surface radius of 1 mm, the sculpture height is sufficiently smaller than that of a surface with a surface roughness of approximately 0.02 to 0.03 micron which is called an optical mirror surface.
However, when a mold for manufacturing a precision diffractive optical element exhibiting a fine curved surface relief configuration where bumps of approximately 1 micron are continued as shown in FIG. 5 is processed on a work 1, a high-precision diamond tipped turning tool 2a is necessary which has a very small point R surface with a radius of not more than several microns. In FIG. 5, the arrow p represents the feed direction of the turning tool 2a. 
However, when a mold is processed by the conventional method by use of the above-described diamond tipped turning tool having an acute point, a high sculpture height is formed on the work surface. In a diffraction lens molded by use of a mold where such a sculpture height h exists, the surface roughness deteriorates to cause light scattering, so that the optical performance deteriorates.
In order that the sculpture height is sufficiently small when the method shown in FIG. 5 is used, it is necessary that the feed amount of the diamond tipped turning tool 2a per rotation should be sufficiently small. However, if the feed amount per rotation is too small, the processing time increases and it is difficult to perform processing with high precision. For this reason, under practical processing conditions, it is necessary to set a certain extent of feed amount, and this makes it impossible to obtain a surface roughness which is no more than that of the optical mirror surface when the conventional processing method is used which uses the diamond tipped turning tool 2a with a point R surface radius of several microns.
Therefore, as shown in FIG. 6, by using a diamond tipped turning tool 2b in which the radius of the point R surface is great to some extent, the sculpture height sh on the processed surface of the work 1 is reduced and the above-described problem can be solved. However, in this case, another problem arises that the stepped configuration of the diffractive surface of the mold for molding a diffractive optical element cannot be formed with precision.
That is, as shown in the enlarged view of FIG. 7, when the radius of the point R surface of the diamond tipped turning tool is great, the stepped configuration of the work surface does not have an acute angle as shown by the thick solid line 1axe2x80x2. The thin solid line 1a represents a stepped configuration of the diffractive surface which is to be realized. Thus, by a method which uses the diamond tipped turning tool 2b having an increased point R surface radius, although the optical performance deterioration due to the light scattering of the molded diffractive optical element is reduced, the point R surface configuration of the diamond tipped turning tool 2b cannot be prevented from remaining at the corner of the diffraction grating.
Since the configuration is transferred to the actual lenses, a great R surface is formed on a corner portion of the diffractive optical surface of the lens and the light incident on the portion does not contribute to the diffraction but becomes zero-order light, so that a needless ghost image appears and the diffraction efficiency deteriorates. As a result, the optical performance is degraded.
An object of the present invention is to provide a method of manufacturing a mold having a micron-order fine diffractive configuration and being excellent in surface precision by paying attention to a turning tool configuration with which a stepped configuration of a diffractive surface can be processed and to a diffraction grating transmissive surface curvature radius and a grating pitch which are particular to a diffractive surface that acts as a lens, and to provide a diffractive optical element with excellent performance by the method.
To achieve the above object, according to one aspect of the present invention, a diffractive optical element that acts as a lens is provided with a diffraction grating having a plurality of ring-shaped ridges, each of the ridges having a transmissive surface whose sectional profile is composed of a plurality of successively connected straight lines of an identical length, the length varying from one ridge to another.
According to another aspect of the present invention, a diffractive optical element that acts as a lens is provided with a diffraction grating having a plurality of ring-shaped ridges, each of the ridges having a transmissive surface whose sectional profile is composed of a plurality of successively connected arcs of an identical radius. In this diffractive optical element, the number of the arcs composing the sectional profile of the transmissive surface of one ridge depends on the radius of those arcs, on a radius of curvature of the sectional profile of the transmissive surface as determined by a phase function, and on a grating pitch and a grating height at that ridge.
According to still another aspect of the present invention, in a method for manufacturing a mold for molding a resin-molded product that has on its surface fine ridges and grooves of which at least one has a sectional profile composed of a plurality of straight lines having different inclinations, the mold being processed by turning by use of a turning tool that has, at its point, a surface whose sectional profile describes an arc of a radius of a few microns and, continuously at both ends of this surface, surfaces whose sectional profiles are linear, those molding surfaces of the mold which are used to form a ridge or groove of the resin-molded product that has a sectional profile composed of a plurality of straight lines having different inclinations are formed by processing those molding surfaces by turning by use of the surfaces having linear sectional profiles of the turning tool, with the turning tool held at a different angle for each of the molding surfaces to be formed.